Transport property analysis method for thermoelectric materials: material quality factor and the effective mass model

Thermoelectric semiconducting materials are often evaluated by their figure-of-merit, zT. However, by using zT as the metric for showing improvements, it is not immediately clear whether the improvement is from an enhancement of the inherent material property or from optimization of the carrier concentration. Here, we review the quality factor approach which allows one to separate these two contributions even without Hall measurements. We introduce practical methods that can be used without numerical integration. We discuss the underlying effective mass model behind this method and show how it can be further advanced to study complex band structures using the Seebeck effective mass. We thereby dispel the common misconception that the usefulness of effective band models is limited to single parabolic band materials.Comment: 5 pages, 3 figure

Memoir: A Collection of Short Stories

This is a collection of short stories derived from the author\u27s experiences at the United States Naval Academy

How University Endowments Respond to Financial Market Shocks: Evidence and Implications

Endowment payouts have become an increasingly important component of universities’ revenues in recent decades. We test two leading theories of endowment payouts: (1) universities smooth endowment payouts, or (2) universities use endowments as self-insurance against financial shocks. In contrast to both theories, endowments actively reduce payouts relative to their stated payout policies following negative, but not positive, shocks. This asymmetric behavior is consistent with “endowment hoarding,” especially among endowments with values close to the benchmark value at the start of the university president’s tenure. We also document the effect of negative endowment shocks on university operations, including personnel cuts.

Charge-transport model for conducting polymers

The growing technological importance of conducting polymers makes the fundamental understanding of their charge transport extremely important for materials and process design. Various hopping and mobility edge transport mechanisms have been proposed, but their experimental verification is limited to poor conductors. Now that advanced organic and polymer semiconductors have shown high conductivity approaching that of metals, the transport mechanism should be discernible by modelling the transport like a semiconductor with a transport edge and a transport parameter s. Here we analyse the electrical conductivity and Seebeck coefficient together and determine that most polymers (except possibly PEDOT:tosylate) have s = 3 and thermally activated conductivity, whereas s = 1 and itinerant conductivity is typically found in crystalline semiconductors and metals. The different transport in polymers may result from the percolation of charge carriers from conducting ordered regions through poorly conducting disordered regions, consistent with what has been expected from structural studies